1. Field of the Invention
The field of the present invention is tandem type vacuum boosters for use in a brake device or a clutch device, and more particularly, tandem type vacuum boosters of a type comprising a booster shell including a partition plate mounted therein for partitioning an interior of the booster shell into a front shell chamber and a rear shell chamber, a front booster piston provided to divide the front shell chamber into a fore-side front vacuum chamber and a back-side front working chamber, a front diaphragm overlapped on a rear surface of the front booster piston, a rear booster piston provided to divide the rear shell chamber into a fore-side rear vacuum chamber and a back-side rear working chamber, a rear diaphragm overlapped on a rear surface of the rear booster piston, a piston boss extending through the partition plate and connected to an output rod, the front booster piston and the front diaphragm as well as the rear booster piston and the rear diaphragm being air-tightly connected respectively to front and rear portions of the piston boss, a valve cylinder continuously formed with a rear end of the piston boss and slidably supported by a rear wall of the booster shell, an input rod disposed within the valve cylinder for advancing and retreating movements, and a control valve also disposed within the valve cylinder for placing the two working chambers into alternate communication with the atmosphere and the two vacuum chambers in response to the advancing and retreating movements of the input rod.
2. Description of the Prior Art
Such a tandem type vacuum booster has already been proposed by the present applicant and is known, for example, from U.S. Pat. No. 4,905,573.
In the above tandem type vacuum booster, in order to bind the front and rear booster pistons and the front and rear diaphragms to the piston boss, three components: the piston boss, an end plate of a cylindrical connecting member connected to the front booster piston and overlapped to a front end of the piston boss, and a retaining plate overlapped to a rear end of the piston boss are coupled to one another by a plurality of through-bolts, thereby fixing the front booster piston and the front diaphragm between the end plate and the front end of the piston boss and fixing the rear booster piston and the rear diaphragm between the retaining plate and the rear end of the piston boss.
In the above prior art tandem type vacuum booster, however, there is a problem that it is necessary to pass the plurality of through-bolts through the piston boss, the end plate and the retaining plate and tighten them, resulting not only in an increased number of parts but also in such labor and time required for assembling the parts.
Thereupon, there are tandem type vacuum boosters known from U.S. Pat. Nos. 4,716,814 and 4,718,328, wherein a sleeve is fitted over an outer periphery of a piston boss and retained at its front end by a nut, and front rear booster pistons and front and rear diaphragms are air-tightly fixed to front and rear portions of the piston boss through the sleeve, respectively, thereby providing an improvement in assembling property with the conventionally used through-bolts eliminated.
With the above construction, however, if the nut should be removed from the piston boss, the sleeve may be moved axially, resulting in a fear that the front and rear booster pistons and the front and rear diaphragms are separated from the piston boss to impair the function of the vacuum booster.